三维城市地表反射率计算模式

地表反射率是控制地表能量平衡的一个重要参数。城市建筑物的分布具有较大的不均一性，这不仅给地基观测城市地表反射率带来了很大困难，数值模拟城市地表反射率也是非常困难的。作者开发了一个三维城市地表反射率模式city_photo，该方法结合了蒙特卡洛方法和几何光学方法的优点，具有较高的精度和较快的计算速度。通过引入城市地图的概念，该模式能够计算具有不同结构的城市的地表反射率。2002至2004年晴空MODIS（MODerate Resolution Imaging Spectroradiometer，中分辨率成像光谱辐射计）1~7通道可见光和近红外通道地表反射率资料被用来检验模式的有效性，位于北京朝阳区的中国科学院大气物理研究所的AERONET站点观测得到气溶胶光学特性和水汽资料，6S（Second Simulation of Satellite Signal in the Solar Spectrum）大气辐射传输模式被用来对其进行大气订正。模式计算的北京城市地表反射率个例与MODIS 7个通道地表反射率观测结果具有很高的相关性，相关系数在0.80~0.93之间，说明模式能够较好地模拟城市地表反射率随太阳和观测角度的变化情况。最后讨论了城市结构对城市地表反射率的影响。

A Three-Dimensional Model to Calculate Surface Reflectance over Urban Areas

The surface reflectance is an important parameter controlling surface energy balance.However,the heterogeneity of the distribution of urban buildings makes it difficult to collect in-situ measurements of surface reflectance.Therefore,there are few studies published on the measurements of urban reflectance(albedo) although there are a large amount researches on natural surfaces,such as crop,forest and desert.The complex urban canopy makes it difficult to simulate urban surface reflectance because of the difficulty in describing the urban configuration.Therefore,the urban areas are not well presented in the current land,atmospheric and climate models.This issue is addressed in this paper.In this paper,a three-dimensional model is proposed,which is of high accuracy and high speed by combining the advantages of Monte-Carlo method and geometrical optical method.The model can simulate the surface reflectance over different urban configuration by incorporating "city map".The MODIS(MODerate Resolution Imaging Spectroradiometer) bands 1-7 reflectance measurements during 2002-2004 are selected to validate the model.The MODIS TOA(top-of-atmosphere) reflectances are corrected for the atmospheric effect using the data of aerosol and water vapor collected by Beijing AERONET site with 6S(Second Simulation of Satellite Signal in the Solar Spectrum) atmospheric radiation transfer model.The validation results show that the model simulations of the surface reflectances are closely related to the MODIS measurements,with the correlation coefficients varying from 0.80 to 0.93,which means that the model proposed here can simulate the influence of solar and view angle on surface reflectance over urban areas.Finally,the influences of the urban configuration on the surface reflectance are discussed.